Wave signaling system



uy 99 l935 vv. A. MacDoNALD WAVE SIGNALING SYSTEM 5 Sheets-Sheet l Original Filed Feb. 15, 1951 BY @wud/MAI ATTORNEYS uy 9, E935.

W. A. MEQDONALD WAVE SIGNALING SYSTEM Original Filed Feb.- ll5, 1931 HEZA 7"/ VE AMPL /F/CA 770A/ I l l lll 5 SheeS-Sheet 2 I i l 500 1000 1500 ATTORNEYS `Fully 9, E935.

W. A. yMQICDONALD wAvEsIGNALING SYSTEM Original Filed Feb. 13, 1951 3 Sheets-Sheet 3 @11Min, ATTORNEYS Feb. 13, 1931, now

Patented July 9, 1935 UNITED STATES `PATENT lOFFICE WAVE SIGNALIN G SYSTEM William A. MacDonald, Little Neck, N. Y., as-

signor to Hazeltine Corporation, a corporation ol' Delaware 29 claims. (ci. 25o-2o) 'I'his invention relates to radio signaling and more particularly to radio receivers of the superheterodyne type. 'I'his application is a division of my copending appiicationserial No. 515,528, filed Patent 1,881,235, grantedOctolber 4, 1932. The principal objects of this invention are to obtain in a superheterodyne type of radio receiver, high sensitivity, selectivity, uniformity of amplification, freedom from cross-talk and interfering noises and whistles and negligible radiation of oscillations, over a wide range of frequency.

To these ends, provision is made for the use of a radio-frequency oscillator, the output voltage of which may bemade to be substantially uniform,v or to vary in a desired manner over the required tuning range of frequencies, the chosen type of variation to be determined by the charac- .teristics of the receiver. Further provision is made for uniformly amplifying and selectively transmitting through the receiver all the important frequencies of the side bands ofthe -received signal.

The usual superheterodyne receiver arrangement comprises an antenna and ground, or a loop antenna, a radio-frequency amplifier provided with signal selecting circuits, a modulator or first detector, a local oscillator for producing local oscillations which are combined with the incoming carrier signal frequency at the modulator to produce a new carrier signal wave of intermediate frequency, an intermediate frequency amplifier, a second detector for `producing the modulation component of the intermediate frequency waves, an audio-frequency amplifienand asignal translating or sound producing system.

It has long been known that the conventional type of antenna and radio-frequency amplifier system, employing as coupling circuits ordinary tuned radio-frequency transformers having comparatively few turns on the primary and a larger number of turns on the secondary winding thereof, is characterized by a response, or amplification, which is considerably greater at the higher than at the lower portion of the tuning range of frequencies. f

'I'he conventional type of local oscillator ordinarily employed in a superhetero'dyne receiver to beat with the incoming signal carrierfre quency delivers an output voltage which also is considerably greater at thel higher frequencies than at the lower frequencies to whichthe oscillator is tuned.

The beat or intermediate frequency voltage output of the modulator, the frequency of which is ordinarily the difference between the radiocarrier frequency and the local oscillator frequency, is proportional to the product of the voltages of the radio-frequency signal and of' the to and including the first modulator, substantially uniform over the operating range of frequencies. In accordance with the present invention there is provided a local oscillator having its circuit constants so proportioned with respect to the characteristics of the antenna and radiofrequency amplifier circuits that the output voltage characteristic of the oscillator compensates for non-uniformity of amplification of the amplifier; that is to say, the output voltage of the oscillator decreases with increased frequencies to compensate for the rising output voltage of the conventional type amplifier at the higher frequencies, whereby the overall response of the receiver is maintained substantially constant.

The oscillator may, of course, be given any de sired predetermined output characteristic; the desired characteristic will be determined by the characteristics of the particular apparatus employed. For example, if the radio-frequency amplifier includes a coupling system of the type described in U. `S. Patent No. 1,763,380, granted to Carl E. Truhe, June 10, 1930, the response of the amplifier may be substantially uniform over'the tuning range of frequency. In such case it will usually be desirable to provide an oscillator having a uniform voltage output over the frequency range of the oscillator.

It is to be preferred that the radio-frequency system be highly selective, by virtue of which extraneous signals are highly attenuated, so that the tendency toward undesired beats in the modulator is further reduced.

An important feature of the invention is the provisionI of an intermediate frequencyamplifier which selectively and uniformly transmits the intermediate carrier frequency and the associated sidebands. The intermediate frequency amplifier comprisesv one or more vacuum tube amplifier stages. coupled by an arrangement of coupling circuits which act to provide the above noted desirable transmission characteristic. One of the interstage coupling circuits comprises a plurality of syntonously tuned circuits coupled by a degree of coupling greater than optimum, whereby the transmission of that particular coupling system is characterized by resonances sufficiently spread to provide a transmission band which readily transmits all the frequencies of the sidebands. Since this type of coupling system provides a somewhat decreased transmission over a range of frequencies between the resonances, other of the interstage coupling systems are proportioned to have a single resonance peak which lies between the resonance peaks of the first-mentioned coupling system, whereby the cumulative effect upon the entire intermediate-frequency amplifier is to provide a substantially uniform transmission over the entire sideband range of frequency.

The interstage coupling systems of the intermediate-frequency amplifier which are characterized' by a single resonance peak, preferably each comprise a transformer having a primary winding and a secondary winding coupled sufiiciently close so that a condenser connected across one of the windings tunes the system as a whole to the intermediate frequency. It is preferable, although not essential, that the winding which is so tuned is the anode circuit, or primary winding, since the input impedance of the coupling system is thereby made relatively high and great attenuation of undesired signals is obtained. The other winding of the transformer is preferably naturally resonant below the range of radio-frequency signals to be received, that is, it is capacitively reactive to frequencies in the broadcast range.

A feature of the intermediate-frequency transformer construction is the adjustment of the voltage step-up ratio to a desired value. This involves providing the proper number of secondary turns to give the desired ratio, while at the same time maintaining the natural resonance of the winding at a desired value below the tuning range of frequency by furnishing the necessary effective capacity. To secure this required effective capacity a proper form factor" -should be chosen for the winding which will provide a distributed winding capacity of the desired value; if necessary, the winding capacity may be supplemented by an external or added capacity.

The combination of a sharply selective radiofrequency amplifier and the intermediateirequency amplifier having the broad, uniform, transmission characteristic is particularly advantageous, since it permits a moderate amount of mistracking of the received signal frequency and of the local oscillator frequency without seriously imparing the quality of transmission through the intermediate-frequency amplifier.

An effective means for. furnishing the desired oscillator characteristic comprises a vacuum tube oscillator having an anode circuit which is coupled to the grid circuit by a dual element coupling system, one of the coupling elements of which tends to produce an output voltage which increases at higher frequencies, the other of which tends to cause the output voltage to decrease at the higher frequencies. By the proper relative proportioning of these two coupling elements any desired output characteristic may be obtained within wide limits.

In addition to the advantage of enabling a desirable response-frequency characteristic to be obtained, this type oscillator affords the additional advantage of generating a smaller amplitude of harmonics, particularly, at the higher frequencies. than the conventional type oscillator. A reduction of harmonics is an important item in a superheterodyne receiver since harmonics, if present to an appreciable degree, beat with extraneous signals in the modulator, and any of these beats occurring within the intermediate frequency transmission band produces disturbing notes.

It is contemplated that the oscillator shall have a relatively small degree of coupling between the grid and anode circuits; by virtue of the small coupling impedance, small changes in the constants of the tube produce only slight changes in the generated frequency.

Other features relate to the shielding of the radio frequency apparatus and connecting leads for the purpose of preventing radiation therefrom.

The above and other features will more fully appear from the following detailed description whenvread in conjunction with the drawings, of which:

Fig. 1 illustrates a superheterodyne radio receiver the component electrical units of which are arranged in accordance with this invention;

Fig. 2 illustrates response characteristics of the radio-frequency amplifier and of the oscillator of the receiver of Fig. 1;

Fig. 3 illustrates transmission characteristics of the intermediate-frequency amplifier and of portions thereof;

Figs. 4 and 4A show a construction of an intermediate-frequency transformer adapted to provide a relatively large voltage amplification;

Figs. 5 and 5A show a construction of an intermediate-frequency transformer adaptedfto provide a smaller voltage amplification than thetransformer cf Figs. 4 and 4A;

Fig. 6 illustrates a superheterodyne receiver embodying the invention but which differs in some respects from the arrangement of Fig. 1, and which is shown complete with a source of power supply;

Jig. '7 shows graphically the types of response characteristics obtainable in the receiver of Fig.v 6; and

Fig. 8 shows in cross-section a coupling device employed in such a receiver.

Fig. 1 illustrates a complete superheterodyne receiver embodying features of the present invention. 'I'he receiver includes an antenna circuit I0 coupled to a radio-frequency amplifier designated in general as II. The radio-frequency amplifier II comprises a vacuum tube amplifier I2 of the four-electrode, or screen-grid, type which comprises a cathode I3, an anode I4, a control electrode or grid, I5, and a screen-grid IB partially surrounding the anode. The amplifier tube I2 is coupled to the antenna through a radio-frequency transformer II, the primary winding I8 of which is connected in the antenna circuit and the secondary winding I8 of which is connected to the grid I5 of the amplifier I2. The coupling transformer I'I is tuned to the signal frequency, or signal-representing oscillations, by means of a variable tuning element in the form of a condenser 20 shunted across the secondary winding I9.

The output of amplifying tube I2 is coupled to a vacuum tube modulator tube 2 I, also of the fourelectrode type, comprising a cathode 22, an anode 23, a control electrode, or grid, 24, and a screengrid 25. The grid, or input circuit, of the modulator is coupled to the anode of the radio-frequency amplifier I2 by a radio-frequency transformer 26, the primary winding 21 of which Vis connected to thc anode I4 and the secondary winding 28 of which is connected to the grid 24. The coupling transformer 26 may be similar to the antenna circuit transformer I1,'and is similarly tuned by a variable condenser 29 shunted across the secondary winding 28.

The receiver is provided with a local oscillator system 30 comprising a three-electrode vacuum tube 3| and associated circuit elements which are so proportioned as to cause the tube 3| to produceoscillation of a desired frequency. The oscillator system constitutes a contributing feature of the present invention and will be subsequently described in greater detail. At this point it is suflicient to state that the oscillator system is provided with a variable tuning element in the form of a condenser 94, and an output coll 32 which is connected in the input circuit of the modulator 2| by virtue of its connection between the modulator cathode 22 and ground. The oscillator is therefore inductively coupled to the modulator.

The anode circuit of the modulator 2| is coupled to an amplifier designated generally as 40, which is adapted to transmit a frequency band which is lower in the frequency scale than the signal frequencies transmitted vby the radio-frequency amplifier. Since the frequency band transmitted by the amplifier 40 lies between the audio-frequency range and the radio-frequency tuning range,this amplifier is called an intermediate-frequency amplifier. The intermediatefrequency amplifier comprises two screen-grid vacuum tubes 4| and 42 which are the same type as the radio-frequency amplifying tube I2. The first intermediate-frequency amplifier tube 4| is coupled to the modulator 2| through a coupling system comprising two similar tuned circuits 43 and 44. One of these tuned circuits 43 which is connected in the anode circuit of the modulator, includes an inductance 45 and a capacity 46; and the other of the tuned circuits 44, which is connected to the grid'of amplifier 4|, includes an inductance 41 shunted by a capacity 48. The inductances 45 and 41 are ,coupled magnetically.

The second intermediate-frequency amplifier tube 42 is coupled to the first intermediate-frequency tube 4| by an intermediate-frequency transformer 49, the primary coil 50 of which is tuned by a fixed condenser and the secondary winding 52 of which is shunted by a resistance 53. An adjustable tap 54 connects a point of the resistance to the grid of tube 42.

The output of tube 42 is coupled to the input of a detector tube 60 by means of a coupling transformer 55 which may be similar to transformer 49. As the intermediate-frequency amplifier constitutes another feature of the invention it will be dealt with in greaterdetail subsequently.

The second detector 60 is of the two-electrode type commonly known as a Fleming valve; although it is shown in form as a. three-element tube having a cathode 6 I, a plate 62 and a grid 63, the cathode and plate are connected together to constitute a single cathode, so that the tube is in effect a two-electrode detector having a cathode and an anode, the grid acting as an anode in this case. The input of the detector is coupled to the output of the intermediate-frequency amplifier by a connection between the high potential end of the secondary of transformer 55 and the anode (in this case the grid) of the detector and by aconnection from the low potential end of the said secondary winding, through a resistance 65, to the cathode. Theresistance 65 is shunted by a condenser 64 for providing a low `impedance path for the intermediate frequency signals around resistance 65.

An audio-frequency amplifier, designated generally as is connected to the detector circuit in the following manner; A resistance 66 is connected at one end to the poin't between resistance 65 and the secondary winding of transformer 55. Thel other end of vresistance 66 is connected through a blocking condenser 61 to one end of a potentiometer, or tapped resistance 68, the other end of which is connected to the cathode 6| of the detector. amplifier is the voltage between the low potential end of resistance 68 and the tap.

The audio-frequency amplifier 1II comprises three stages of amplification.v The first two stages include respectively amplifying tubes 1| and 12, resistance-coupled in tandem in a conventional manner by shunt resistances 16 and 11 and blocking condenser 18. The output of tube 12 is coupled to the last audio amplifying stage, which comprises a pair of tubes 13 and 14 connected in the well-known push-pull relation.v The output of the push-pull stage is coupled to a loud speaker 15.

The receiver is adapted to be tuned by a unicontrol arrangement; this is effected by operating the adjustable element, or variable condensers 20, 29, and 94 from a single shaft; this operation is represented by the dotted lines 56 and 51.

The sources of operating potentials such as the filament heating sources and the grid, screengrid and anode potentials, are not shown in the drawings. These'potentials may be supplied by any of the well-known methods. There are indicated in the drawing potentials which are well adapted for application to the various leads; these potentials are given with respect to ground potential. A

, For the purpose of automatically controlling the" strength of the signal current delivered to the audio amplifier, there is provided a volume controlling system which automatically regulates the amplification of the receiver so that the detected, or audio, signals remain substantially uniform. The volume controlling arrangement is of the type described in a paper presented before the Institute-of Radio Engineers by H. A. Wheeler and published in pages 30-34 of the Proceedings of the Institute of Radio Engineers, January, 1928. The'system comprises a connection 88 extending from the lower end of resistance 66 to the control electrode I5 of radio-frequency amplifier I2 and to the control electrode of intermediate-frequency amplifier 4|. The connection 8|) is led to the control electrodes of amplifiers I2 and 4| by connections' to the low-potential ends of the grid circuit windings I9 and 41, respectively, of the associated coupling systems. Thereare included in the connection/8|) resistances 8| and 82, the function of which will be more fully explained later. For the purpose of keeping the grid potential from the cathodes, but still enabling the grid circuits to. be completed, there are provided blocking condensers 83 and 84. The above-described automatic volume control system is disclosed and claimed in H. A. Wheelers copending application Serial No. 203,879, filed July The input potential for the audio 7, 1927, and in his U. S. Patent 1,879,863, issued Sept. 27, 1932.

The receiver is provided with a number of resistors, some of which furnishv biasing potentials for vacuum tube grids, and others of which are inserted in the leads supplying operating potentials to the electrodes of the tubes. There are also provided by-passing condensers at advantageous points. These elements contribute to-` ward good operation; and since they are in general use and are well-known in the art, no further details are given here.

The following is a brief description of the operation of the receiver: A radio signal received by the antenna l0 is selected in the well-known manner by the signal selector composed of the selective circuits of coupling transformers |1 and 29, which are tuned to the same frequency. The modulated carrier signal, after being amplified in the radio-frequency amplifier is impressed upon the grid circuit of the modulator 2|. The oscillator 30 is tuned in conjunction with the-selective circuits and 29 to supply to the modulator a frequency which differs from the signal frequency by a desired amount. The oscillator frequency may be either greater than, or less than, the radio carrier frequency, but it is preferably greater than the radio frequency. By virtue of the well-known phenomenon of modulation, there is produced in the output of the modulator a new carrier frequency which is equal to the difference between the frequency of the received radio signal and the local oscillator frequency. This difference in frequency is commonly known as the intermediate carrier frequency, since it is lower than the radio-frequency of the received signal but is above the audible range. The intermediate carrier frequency has associated with it the side band frequencies with which the signal is modulated.

The selective coupling circuits of the intermediate-frequency amplifier are adjusted to freely transmit the intermediate carrier-frequency and the associated side bands and to effectively exclude all other signals. The amplified output of the intermediate frequency amplifier is impressed upon the two-element detector 60, in the output of which there appears the modulation component, that is, the audiofrequency signals. The audio-frequency signals are amplified in the audio-frequency amplifier in the well-known manner and are converted into sound by the loud speaker 15 connected to the output of the audio amplifier.

The following is a brief outline of the operation of the volume controlling circuit:

When a modulated carrier signal is impressed upon the two-electrode detector, there appears across resistances 65 and 66 a voltage having two components, one an audio-frequency component and the other a. direct current component. The direct current component is proportional to the strength of the received carrier signal. An increase of the received carrier signal has the effect of increasing the voltage across resistances 65 and 66, that is, of causing the potential of point 95 to become more negative with respect to ground. Since the potential at point is impressed through the connection upon the grids of amplifying tubes |2 and 4|, the effect is to render these grids more negative when the signal strength increases. Likewise when the signal strength decreases, the grids of amplifiers |2 and 4| become less negative. Due to the variation of the potential of the amplifier grids in this manner,

attendant upon the variation of the received signal strength, the amplification increases whenthe signals are weak and decreases when the signals are strong, so that the net effect is to maintain the signals at the second detector substantially uniform in strength. The audio-frequency component of the detected signal is prevented from appearing at the grids of amplifiers I2 and 4| by virtue of the filtering action of resistances 9| and 92 and condensers 94 and 83.

The magnitude of the `intermediate frequency voltage at the output of the modulator 2| is expressed by the relation:

Where El is the intermediate frequency voltage at the output of the modulator;

E, is the radio-frequency signal voltage at the input of the modulator;

Eo is the voltage of the local oscillator at the input of the modulator; and

k is a proportionality constant.

It is generally desirable to maintain constant the amplification of the portion of the receiver between the antenna and the intermediate frequency amplifier, that is to maintain constant the product ES.E0. It is contemplated to provide this feature of uniform amplification by means of a novel combination of the local oscillation generating system 30 and the radio-frequency amplifier. The elements of the oscillator system are so proportioned with respect to the response characteristic of the radio-frequency amplifier, that the oscillator voltage E0 varies in a manner complementary to the variation of the amplified radiofrequency signal voltage ES, whereby the product EaEc and hence the voltage E1, is maintained substantially constant; or if desired, El may be given any desired characteristic.

Referring now specifically to the oscillator system, it comprises a three-electrode oscillating vacuum tube 3| having a cathode 90, an anode 9| and a control grid 92. 'I'he grid circuit includes an oscillatory circuit comprising an inductance 93 and a variable capacity 94. 'I'he inductance 93 is connected at one end to the grid 92 and at the other end through the parallel arranged capacity 95 and resistance 96 to ground. It will be seen that the coupling capacity 95 is in series with the variable capacity 94 and the inductance 93. There is shunted across capacity 95, a small variable capacity |06 for enabling a propfe'r adjustment to be obtained. The cathode is grounded through a parallel arranged resistance 91 and capacity 98. The function of the resistance 91 is to provide a biasing potential for'the grid.

For the purpose of establishing the condition of oscillation, the anode 9| is connected through a coil 99 to the point between the grid circuit inductance 93 and capacity 95. The coil 99 is so situated relative to coil 93 that a substantial degree of inductive coupling exists betweenthe two coils. By virtue of this feedback circuit arrangement of the oscillator there exist in common with both the grid, or input, circuit and the anode, or output, circuit of the thermionic oscillator tube, the capacity 95 and the mutual inductance M of coils 93 and 99. These common, or mutual impedances are made large enough to produce suii- This circuit arrangement is seen to comprise, in series, at the input system of the oscillator tube, the capacity 94, the inductance 93, and the fixed capacities 95 and 98, through the ground connection. 'I'he connection from the anode 9| to the cathode 90 through the inductance 99 meets these series arranged elements of the input system at a point between two of the -plurality of series arranged capacities; that is, this junction point is located between condenser 94 and 95 (inductance 93 intervening).

Since the voltage across the capacity 95 is least at the highest frequencies of the range over which the oscillator is tuned, and is greatest atthe lowest oscillator frequency, it follows that the effect of this capacity in producing an oscillating voltage is greater at the lower frequencies than at the higher frequencies. It further follows that the effect of the mutual impedance M in producing an oscillatory voltage is greater at high frequencies than at lower frequencies.

The oscillators commonly employed heretoforein superheterodyne receivers have been of the 1 conventional type, that is, a simple feed-back coil` has been used to transfer energy from the anode circuit to the grid circuit. As already observed, however, this form of oscillator is characterized by an output voltage which increases in magnitude when the oscillator is tuned to higherfrequency; which is usually an undesirable characteristic. Bymeans of the present oscillator, however, it is possible to obtain any desired output characteristic within wide limits. Assume, for example, that there is employed the present type of oscillator system, and that the output voltage at the higher frequencies is greater than desired. Accordingly, if the mutual inductance be decreased relative to the mutual capacity, as by removing turns frorn coil 99, the output voltage of the oscillator is reduced at the higher frequencies to a greater degree than at the lower frequencies. By properly proportioning the mutual inductance and mutual capacity which couple the input and output circuits of the oscillator tube 3l, any desired output voltage-frequency characteristic may be obtained; and hence any, desired responsefrequency characteristic may be( imparted to the received.

The radio-frequency coupling systems I1 and 26 may be transformers having primary windings of relatively few turns and secondary windings of many more turns. 'I'he effect of employing such transformers in an amplifier is to cause the amplifier to amplimy high-frequency carrier signals to a much greater extent than lower frequency carrier signals. The amplification-frequency characteristic of such an amplifier is illustrated graphically by curve a of Fig. 2. Fig. 2 is a graph, the ordinates of which represent the ratio of maximum to minimum amplification and the absciss of which represent the frequency of radio-frequency carrier signals. The type of transformer indicated by curve a provides an amplification which is almost three times greater at a frequency of 1500 kilocycles per second than at 500 kilocycles per second. The oscillator output, then, in order to compensate for this characteristic ofthe radio-frequency amplifier must be given a characteristic of the type indicated by curve "b of Fig. 2. This type of oscillator output characteristic may be obtained by properly proportioning the relation between the mutual inductance and the mutual capacity of the oscillator circuit, in the manner previously described. 'Ihe overall characteristic due to the combination of the amplifier having the characteristic (a) and the oscillator having the characteristic (b) is represented by curve (c): which is substantially constant over the tuning range. It should be understood that these curves represent the law of variation of the individual responses rather than the relation of one response to another. For it will be clear that while the magnitude of the oscillator response characteristic may be fixed, the magnitude of the response to the incoming signal depends upon the strength of the particular signal being received. This assumes, of course, that the automatic volume control effect upon the radio-frequency amplifier alone is not made absolute; some of theautomatic volume regulation takes place in the first intermediate-frequency amplifier in the receiver of Fig. 1. Likewise, the magnitude of the response characteristic (c) is dependent upon the proportionality factor (lc) Although a conventional type transformer may be employed to couple the antenna to the radiofrequency amplifier, it is preferable to employ a coupling system of a type inwhich the transformer comprises a helical secondary winding Wound on a cylindrical core, and a compactly wound primary winding of a large number of turns, located near one end of the secondary winding. 'Ihe primary coil is preferably wound in the oppositev direction to the windings of the secondary coil, the directions of the windings being considered for this purpose relative to the low-potential terminals. The effect of this type of transformer `is to render the antenna circuit inductive over the entire broadcast range of frequency, whereby the voltage ,-.ampliflcation is maintained substantially uniform over that range. The above-described coupling system is described and claimed in my copending application Serial No. 280,464, filed May 25, 1928, and the transformer per se is disclosed and claimed in my copending application Serial No. 498,785,

filed November 28, 1930,'n0W Patent 1,892,354,V

granted December 27, 1932.

If the antenna coupling system is not of the uniform response type, the oscillator will have to be proportioned to compensate for this irregularity in addition to compensating for the nonuniformity of the radio-frequency amplifier.

In additiony to enabling the amplification characteristic of the portion of the receiver ahead of the intermediate frequency amplifier to be adjusted to provide uniform amplification over a broad range of tuning frequencies, the use of an v oscillator circuit of the type employed in the receiver of Fig. 1, provides additional advantages over oscillator circuits previously employed.

One of these advantages lies in the fact that when the oscillator circuit is proportioned to provide an output voltage which is constant or which increases at lower frequencies, the effective coupling between the grid and anode of the oscillator` is relatively small at the higher frequencies to which the oscillator may be tuned. The reason `for the small degree of coupling at the higher frequencies is that the reactance of the coupling condenser becomes small at these frequencies. This is especially important because the smaller the coupling between the grid and plate circuits, the smaller is the effect of the tube constants upon the generatedfrequency; or expressed in another manner, changes in the tube constants such as'the mutual conductance or plate resistance, produce a small or negligible effect upon the generated frequency.

It is vhelpful to be able to adjust the coupling between the grid and anode circuits of the oscillator to the value that will produce optimum coupling and the desired output voltage at the highest frequency to which the oscillator will be tuned; for it is in the high-frequency range that very small changes in the electrical constants of the oscillator produce relatively large changes in the generated frequency.

At the lower frequencies, a given change in the constants produces a smaller effect upon the oscillator frequency than at the higher frequencies. Consequently, at the low-frequency range of operation the effective grid-anode circuit coupling may be made much greater than at the high-frequency range; and even though there may be appreciable changes in the tube constants at the low-frequency range, the effective change in the generated frequency is smaller.

A second major advantage, and an important reason for the use of the-oscillator circuit of the type described. is the small amplitude of the harmonic frequencies produced. It has been found that the harmonics produced by an oscillator of the conventional type may be much greater at the high-frequency range of operation than at the low-frequency range. In the conventional type of oscillator system, the large magnitude of the harmonic frequencies at the high-frequency range is accentuated by the fact that in order to cause the tube to oscillate at the lowfrequencyI range, it is necessary to provide a coupling between the grid and anode circuits which is substantially over-optimum at the high-frequency range. This relatively large coupling at the high-frequency range produces a higher output level than at the low-frequency range; and since the harmonics produced occur at high exciting voltages, it has been found that the magnitude of the vharmonic frequencies generated by the conventional type of oscillator may be much higher at the high than at the low-frequency range.- Y

By employing an oscillator circuit of the type of the present invention, it is possible to employ substantially optimum coupling between the grid and anode circuits of the oscillator over the entire frequency range of operation, or, if desired, opti- Vmum coupling may be provided at the highfrequency range and the coupling impedance automatically increased at lower frequencies.A

Although the magnitude of the harmonic frequencies increases when the grid-anode coupling is increased, and also as the output level is increased, vthere is an opposing action which tends to reduce the magnitude of the harmonics at the lower frequencies. This opposing action results from the increased size of the tuning capacity at the lower frequencies.

By a proper choice of the signal level at the input of the modulator, that is, by providing sufficient radio-frequency amplification, so that a very large oscillator voltage is not required to provide a given intermediate frequency voltage at the output of the modulator, and by causing the output voltage of the oscillator to remain unlform or to decrease with increased frequency, it 'is possible to cause the magnitude of the harmonic frequencies to remain substantially uniform over the entire range of operation and at a sumcientlylow level so that they do not seriously interfere with the normal performance of the receiver.

The electrical constants of the elements associated with the oscillator system 30 have been so chosen as to provide unicontrol operation between the radio frequency and oscillator circuits` with a fixed frequency difference between these circuit. This result is accomplished by the proper choice of the inductance 93 and the capacity 95 which modify the action of the tuning condenser 96 of the oscillator frequency determining circuit. The capacity 95 in conjunction with the inductance 83 and the mutual inductance M between coils 98 and 93 regulate the slope of the output vn'characteristic of the oscillator. The capacity 95 therefore functions in the dual sense to proportion the output level and also to align the oscillator circuits with the radiofrequencytuner circuits to provide a constant frequency difference between these systems.

The intermediate-frequency amplifier is so designed that there is provided a fairly uniform transmission of all the frequencies of the side bands of the intermediate-carrier frequency, and furthermore, means are provided for insuring that no frequencies other than those of the desired signal are transmitted to any substantial degree.

The coupling system which couples the output of the modulator to the input of the intermediatefrequency amplifier is of the double-tuned type, that is, it comprises a pair of syntonously tuned circuits coupled electromagnetically, one of the tuned circuits 4l being situated in the modulator anode circuit and the other tuned circuit M in the grid circuit of the first intermediate-frequency amplifier. The degree of magnetic ycoupling is preferably somewhat over-optimum so that the transmission band of this coupling system is somewhat broadened by virtue of the pair of slightly spaced resonant peaks which it is well known are obtained by this arrangement. The frequency spread of the resonance peaks should preferably be about equal to or greater than the frequency range of the side bands with which the intermediate carrier-frequency is modulated.

The two succeeding intermediate-frequency transformers 48 and 55 are preferably identical, although this identity is not essential. These transformers are so constructed that there exists a close electromagnetic coupling between their primary and secondary windings, whereby the coupling system tunes as a whole at the resonant frequency of the winding which is shunted by the condenser. Each of the two transformers 4l and 55 is characterized by a single resonance, rather than a double resonance such as is obtained from the first intermediate-frequency coupling system.

There is an advantage from a commercial standpoint, as well as from a transmission standi pointbin providing one coupling system of the double-resonance type and the other coupling system of the single-resonance type. It has been found difiicult to adjust the double-resonance system commercially so that the two resonance peaks are properly spaced and are of approximately the same height. Hence, it is desirable that there be only one oi these double-resonance systems. The advantage from the transmission standpoint is that the single-resonance of the transformers I9 and UI can be made to lie Yruidway between the peaks of the double-resonance coupling system, whereby the over-all transmission provided by the intermediate frequency ampliiier is substantially uniform over the side band range of frequencies.

Curve a of Fig. illustrates the transmission-frequencycharacteristic of the doubleresonant system. The ordinates represent the ratio of the rgain at resonance to the gain near 75 resonance and the absciss represent frequencies in kilocycles' per second. The frequency marked zero represents the intermediate carrierfrequency, and the absciss on either side of the carrier-frequency are the frequencies of the side bands.

Curve b of Fig. 3 represents the transmissionfrequency characteristic of the transformers 49 and 55. The combination of the two types of coupling system cooperates to produce a. transmission characteristic of the type illustrated 'in curve c of Fig. 3. The intermediate frequency amplifier can be readily constructed so that the over-all response at a side-band frequency of four kilocycles from the intermediate carrierfrequency is as much as 80% of the response at 'the carrier-frequency.

By providing a relatively flat-topped response characteristic of the typel of curve c, slight mistracking between the oscillator frequency and radio-frequency carrier frequency, that is, variations inthe diierence between the radio-frequency carrier and oscillator frequency, cause no serious change in the over-all sensitivity over the important side-band range.

In considering an intermediate-frequency amplifier of the type under discussion including coupling systems utilizing double tuning between the output of one tube and the input of the second tube, together with coupling systems utilizing but a single resonant circuit tuned to the intermediate frequency, it has been found preferable, although not essential, to arrange the coupling systems utilizing but a single physically resonant circuit so that the resonant circuit is connected between the cathode and anode of the preceding amplifying tube rather than between the cathode and control grid of the succeeding tube. Where the very highest order of performance is desired, it is usually desirable to connect the coupling systems of adjacent stages in this way, for such connections tend to attenuate voltages of undesired frequencies more rapidly than when the tuned circuit of the resonant unit transformer is connected between the cathode and control grid.

Conditions of design, however, may require the physically tuned resonant circuit of the intermediate frequency coupling system to be connected between the cathode and control grid. If this be the case, then, the full advantage of band selection can be secured in the intermediatefrequency amplier, but there may be a greater tendency for interfering signals on nearby channels to be transmitted directly through the intermediate-frequency amplifier, thus producing interfering beats by the second detector.

It is usually desirable to tune the 4primary circuit of transformer 55 rather than the secondary circuit because this transformer operates into-a two-electrode type of detector, namely, detector 60. This form of detector imposesa shunt load on the transformer, so that if the secondary circuit were tuned instead of the primary circuit, the effect would be to materially impair the resonance characteristic of this transformer. By

tuning the primary circuit and employing a stepdown transformation ratio, the impedance of the secondary circuit may be arranged to match the impedance of the system into which it feeds, thus producing the most efficient condition of operation.

If for some design reason it is necessary to tune the secondary rather than the primary circuit, then the detector may be connected across a portion of the tuned secondary, the impedance of which is approximately equal to the imped- .ance of the load circuit.

by a physical capacity shall have such elec-- trical constants that it will be naturally resonant at a frequency lower than the lowest broadcast frequency to be received. 'I'he factors which determine the resonant frequency of this winding are: its natural inductance, the distributed capacity of the winding and the capacity of the devices connected across its terminals. All of these elements must be considered in the selection of the proper resonant frequency such that the` winding is always capacitively reacted to frequencies in the broadcast band.

If this precaution is not followed and the untuned winding is arbitrarily chosen so that its natural period falls within the broadcast band, then it has been found that there is usually a high order of amplification to broadcast frequencies, and voltages of signal frequency which would normally be of small magnitude produce, undesirable heterodyne whistles in the receiver.

Intermediate frequency=175 kilocycles per second.

Inductance of primary winding=8.99 millihenries.

Inductance of secondary winding-5.16 millihenries.

Coupling coefficient between primary and sec-l ondary windings=37%.

Capacity across primary winding=100 micromicrofarads approx.

The above inductance values arc obtainable by winding on a one-half inch core in bobbins onequarter inch wide, a primary coil of 800 turns of No. 38 double silk-covered copper wire. The bobbins containing the primary and secondary coils are coaxially placed side by side and are enclosed by a shielding can of 1%-inch diameter.

When the transformer is constructed in accordance with the above specification the resonant period of the secondary 'winding is about 350 kilocycles per second. This value of the resonant frequency is not necessarily the best value for` all receivers; the best value for the resonant frequency will depend somewhat uponl the particular design of the receiver. If it be desired to make-the resonant period higher than the value of about 350 kilocycles per second, the number of turns on the secondary winding should be somewhat less than the value given in the above table; or, on'the other hand, if it be desired to reduce the number of secondary turns and still maintain the resonant frequency at about 350 kilocycles per second, the coil should be constructed to have a higher distributed capacity, in accordance with methods outlined in succeeding' paragraphs. f

It has been found convenient to regulate the amplification of the intermediate-frequency amplifier by means of the turns ratio assigned to the transformer, the resonant frequency of thesectively large and the secondary winding so shaped that it has a relatively low distributed capacity. Factors which tend to reduce the distributed capacity are small wire size, thick insulation over the wire and a thin pancake form of coil.

An intermediate-frequency transformer having a low distributed capacity is illustrated in Figs. 4 and 4A; Fig. 4 being an elevation view in section, and Fig. 4A being an end view. In these figures, the core is a cylindrical form |00, the primary winding is a pancake form of coil and the secondary winding is a thin pancake coil |02. The coils are preferably wound with wire having heavy insulation.

If, however, it is desired that the transformer have a lower amplification than the type just described, the secondary winding may be composed of fewer turns of a heavier wire having thin insulation, and the form of the windings may be such that the axial length is relatively large and the radial thickness small. Such a transformer of low amplification and high distributed secondary capacity is illustrated in Figs. and 5A, in which |03 represents the core, |04 the primary winding, and |05 the secondary winding.

It is usually preferable to construct the intermediate frequency transformer so that the value of the effective capacity required to shunt one of the windings, and thereby tune the transformer to the intermediate frequency, is fairly large, that is, somewhere in the order of 100 micro-microfarads; by virtue of this large capacity, slight changes in the input or output capacities of the associated vacuum tubes will not materially affect the resonant frequency of the transformer.

It is inconvenient from a mechanical and from an economic standpoint to construct an adjustable capacity of the order of 100 micro-microfarads. A form of external condenser which has been found satisfactory comprises a pair of metal leaves about one inch square separated by a di-electric of mica or other suitable material and provided with a supporting frame and a compression screw which may alter the spacing between the metal leaves, thereby providing an adjustment of the capacity value. The capacity value of such a condenser is of the order of 40 micromicrofarads. If a larger value of capacity is desired, it is necessary to either increase the size of the leaves or to add additional leaves. Either of these expedients for providing the increased capacity requires an extremely rugged construction in order to prevent warping of the leaves; which would result in changing the natural period of the transformer and thereby materially affect the sensitivity.

Torpermit the use of the smaller condenser of about 40 micro-microfarads, it is preferable to construct the transformer so that the winding across which this external condenser is connected has a high distributed capacity, in ac cordance with the previous discussion.

It has been found that greater sensitivity is required in certain localities than in other localities. To enable the receiver to meet any of the required conditions of sensitivity, one of the intermediate-frequency transformers is provided with an impedance shunted across its secondary winding so that the sensitivity may be adjusted to4 any definite value. This is the resistance 53 of Fig. l having thc adjustable tap 54. It is preferable to provide two or more definite taps having a switch for selecting anv one of them.

rather than to employ a potentiometer of the slide-wire type.

Fig. 6 illustrates a superheterodyne receiver in accordance with the present invention which varies in details from the receiver of Fig. 1. The potentials required to energize the vacuum tubes of the receiver are obtained from a source of rectified, filtered alternating current. The radiofrequency amplifier is provided with three selective circuits instead of two selective circuits as in the receiver of Fig. l. The antenna is coupled to the first radio-frequency amplifier tube IIB by a preselector composed of two tuned circuits. The first of these tuned circuits comprises a transformer' ||0, the secondary winding of which is tunable by a variable condenser Transformer 0 is preferably constructed in a form similar to that of transformer I1 of Fig. 1, previously described. The primary winding of transformer I0 has shunted across it a high resistance |I2 provided with a variable tap connected to ground, the purpose of this variable resistance being to provide a manual volume control.

The second selective circuit of the preselector comprises an inductance ||3 and a variable condenser ||4. 'I'he second selective circuit is coupled in tandem with the first selective circuit by means of a small inductance coil I5 attached at one end to the low-potential end of the secondary coil of transformer ||0 and at the other end to the low potential terminal of inductance I3. The coupling is effected by virtue of mutual inductance M' which exists between coils ||5 and ||3. This second selective circuit is connected to the input of amplifier tube IIS.

The third selectivc'circuit of the radio-frequency amplifier is the coupling system ||1 electrically located between the output of amplifier H6 and the input of the modulator I8. This coupling system is of the type disclosed in United States Patent No. 1,763,389 issued June 10, 1930, to C. E. Trube. This coupling system comprises an input circuit including the anti-resonant combination of fixed condenser |200 and fixed inductance |2|, in series with an inductance |22. Inductance |2| and condenser |20 should be antiresonant at a frequency which is slightly below the lowest frequency of the broadcast range. The output circuit of the coupling system ||1 comprises an inductance |23 which is tunable to the signal frequency by a variable condenser |24. The inductance |23 is magnetically coupled to inductance |2| and to inductance |22. The effect of such a coupling system is to cause the amplifier toprovide an amplification which is substantially uniform over the tuning range of frequency; this effect is described in greater detail in the above-noted Trube patent.

The radio frequency amplifier of Fig. 6 possesses a two-fold advantage over the radio frequency amplifier of Fig. 1. One of these advantages is the greater selectivity afforded by virtue of the three tuned circuits instead of only two tuned circuits. This results in greater attenuation of signals of undesired frequencies, whereby whistles and other undesirable noises due to the presence of undesired frequencies in the receiver are substantially eliminated.

Because of the great attenuation of undesired frequencies, the magnitude of image frequency signals received at the modulator is very small. By the term image frequency is meant the frequency which added to, or subtracted from, the

local oscillator frequency would produce in the modulator an intermediate-frequency signal of the same frequency that the local oscillator frequency?I combined with the received signal f requency produces in the modulator. More specifically, the image frequency is an undesired frequency which is equal to the signal frequency plus twice the intermediate frequency, when the oscillator is adjusted at a frequency higher than the received signal frequency.

The two selective circuits of the radio-frequency amplifier in Fig. 1 attenuate the image frequency to a low value, but, of course, the attenuation is not as great as in the radio-'frequency amplifier of Fig. 6. Consequently, the amplifier of Fig. 6 is preferable in localities in which many interfering signals are present; but in other localities, Where there is comparatively little interference, the selectivity o f the radio frequency amplifier of Fig. 1 is adequate.

The other advantage of the radio-frequency amplifier of Fig. 6 is that amplification of the amplifier can be made substantially uniform over the broadcast range of frequency.

The local oscillator system |25 is substantially the same as the local oscillator of the receiver of Fig. 1; hence, no further discussion of it is required here except to state that since the radiofrequency amplifier is characterized by a response which is uniform over the tuning range of frequency, the condition required to provide over-all uniform amplification is that the oscillator circuit shall be so proportioned that its output voltage is uniform throughout its range of operation. If desired, of course, the over-all amplification may be given any characteristic desired by proportioning the oscillator circuit accordingly in the manner previously described.

Fig. 7 illustrates graphically the types of response characteristics obtainable in the receiver of Fig. 6. The ordinates represent relative am` plication and the absciss represent frequencies of the tuning range. Curve (a) indicates the over-all response at the output of the modulator, which is obtainable when both the radiofrequency amplifier and the loscillator response characteristics are also of the type of curve (a), that is, uniform with frequency. Curve (b) shows the over-all response at the output of the modulator when the radio-frequency amplifier response is uniform but the oscillator response is made to decrease at the higher frequencies. In this case the oscillator response is also of the sanie general type as that indicated by curve (b).

The receiver of Fig. 6 is provided with a single intermediate-frequency amplifier tube |26, the input of Which is coupled to the modulator ||8 by a coupling system |50. The output of amplifier |26 is coupled to a detector tube |21 by a coupling system The circuit arrangement of the coupling system |50 is similar to that of the coupling system between modulator 2| and intermediate frequency amplifier 4| of Fig. 1. The coupling system comprises a pair of coupled inducta'nces |52 and |53, respectively tuned to the intermediate frequency by condensers |54 and |55. The system |50 may be constructed in the same manner as is the similar system of Fig. 1; it is often preferable, however, for the purpose of easily obtaining a correct adjustment, to proportion the elements of coupling system |50 in the manner to be presently described.

Fig. 8 illustrates in cross-section the construction and assembly of such a coupling system. The arrangement comprises the coils |52 and |53 random Wound, or layer wound, respectively in bobbins |51 and |58. 'Ihe bobbins are mounted coaxially on a form, or core, |59, which is adapted to be fastened to a base, or a chassis, by brackets '|60 and IBI. Surrounding the bobbins con` taining the coils is a cylind'rically shaped shielding ring |62, adapted to be fastened to the base at the lower end. 4The shielding ring, which is preferably of heavy copper or aluminum or other metal having low specific electrical resistance, is located in proximity to the coils so that there exists a substantial coupling between the ring and the coils. l

4 The effect of the shield is to cause the effective coupling between the coils to be reduced to a lower value than would exist if the shield were absent. It is possible,therefore, to place the coils fairly close together so that there exists a higher degree of coupling than is required for the proper performance of the coupling system; the shielding ring is then so placed relative to the coils that the effective coupling between the coils is reduced to the required value. The coupling system of Fig. 8 and the transformer structure thereof are described and claimed in United States Patents 1,855,054, and 1,855,055, issued April 19, 1932, in the name of J. Kelly Johnson.

The coupling system |5| may be similar to coupling system 49 of Fig. 1, except that the fixed tuning condenser |63 is shown shunted across the secondary winding instead of the primary winding, as in the case of Fig. 1.

The detector tube |21 is of the three-electrode type instead of the Vtwo-electrode type shown in Fig. 1. There is shown no automatic volumej controlling circuit such as is shown in Fig. 1. The three-electrode detector could be used with a volume controlling circuit if desired, but it is usually preferable to employ a two-electrode detector for this purpose.

The audiofrequency amplifier is a push-pull stage comprising tubes |28 and |29. The output of the push-pull stage is. coupled to an electrodynamic type of loudspeaker |30.

The energizing potentials for the entire receiver are supplied by a power supply system |3|` which is adapted to be operated from an ordinary commercial source of low-frequency alternating current by means of a conventional socket plug |32. The drawings illustrate a conventional arrangement of a power transformer |33, a full wave rectifier |34, a filter |35 and all the resistors for providing the necessary operating potentials for the various tubes of the receiver. The field winding |36 of the loudspeaker is also operated from the power set.

In any radio receiver employing an oscillator there is a tendency for radiation to occur which may seriously interfere with neighboring receivers. The radiation is caused by the electrostatic and electromagnetic fields of the oscillator tuning coil and of exposed portions of the oscillator leads connecting' the various elements of the oscillator system. Additional radiation occurs from other elements of the receiver because of their association with the oscillator system. 'I'here is'some radiation from the elements and leads of the modulator since the oscillator voltage is` It has been found that by means of adequate shielding it is possible to reduce all radiation 'to such a small degree that other receivers lovention. These disturbances may be largely prevented by shielding the oscillator coil by means of a grounded metallic enclosure of low specific resistances, such as copper or aluminum. This is shown in Fig. 6 as the grounded shield |31. The oscillator tuning condenser and the oscillator tube are also shielded by means of grounded metal enclosures |38 4and |39, respectively. It is preferable to mount all the apparatus on a shallow chassis pan and to have all of the physically large elements such as tubes, coils and tuning condensers mounted on top of the pan. All the connecting leads from the various component elements should be brought through holes in the pan and all connections made beneath the pan. Since certain of these leads carry radio-frequency currents of relatively large magnitude which would tend to radiate, a metal bottom is provided for effectively shielding all leads.

Second order radiation effects resulting from the introduction of the oscillator voltage into the input of the modulator are prevented by providing appropriate grounded shields |40 and |4| for the modulator tube and for the inductances of the coupling system at the input of the modulator.

Radiation is further suppressed by isolating the oscillator circuit from the common power supplies as far as radio-frequency currents are concerned. The principal isolation required is the use of a separate automatic biasing resistor |42 in the cathode circuit of the oscillator and proper filtering for the anode potential supply.

The oscillator voltage delivered to the modulator tube should be prevented from affecting other elements of the receiver bysimilar isolation means; these include separate filtering of the grid-biasing circuit, the screen-grid circuit and the anode circuit of the modulator. It is advisable to shield the radio-frequency tube and the' associated circuit elements and connections, to prevent radiation from these sources; these shielding means are shown in Fig. 6 as shield |43 around tube ||6, and shields |44 and |45 around the inductances of the preselector.

If the previously described precautions as to shielding have been observed, it is usually unnecessary to provide filtering in the power circuit leads. Where it is found necessary, however, to filter the power circuit prior to rectification, this may be accomplished by inserting inductance coils |46 and |41 in series with the power line and a pair of shunt capacities |48 and |49 in series across the line, the midpoint between the capacities being grounded. Although the shields are illustrated only in Fig. 6, it should be understood that they are equally applicable and useful in the receiver of Fig. 1.

What is claimed is:

1. In a radio receiver, in combination, means for receiving radio signals, a frequency selecting system including a variable condenser for tuning said selecting system, a local oscillator system and a modulator for modulating the radio signals with the oscillations from said oscillator, said oscillator system comprising a vacuum tube havlng a cathode, a grid and an anode and circuits connected therewith, the anode circuit being coupled to the grid circuit both capacitively and magnetically to produce said oscillations, the capacitive coupling comprising a condenser in the cathode circuit and common to the grid and anode circuits, a variable condenser connected to tune said grid circuit, and an output circuit from said oscillator coupled to a circuit of said modulator, the variable condenser` of said selecting system being operated in conjunction with the variable condenser of said oscillator grid circuit by a uni-control arrangement, and said coupling condenser and lsaid variable condensers being so related that there is a fixed frequency difference between the frequency towhich said selective circuit is tuned and the frequency to which said oscillator is tuned.

2. In a signal responsive system, a signal frequency amplifier, a modulator coupledV to the output of said amplifier and a vacuumiube oscillator, having a cathode, anode and grid and circuits connected therewith, means for tuning said amplifier and said oscillator over a range in frequency, a uni-control device for simultaneously adjusting the tuning of said amplifier and of said oscillator and means for maintaining a fixed frequency difference between the frequency to which said amplifier is tuned and the frequency to which said oscillator is tuned over 'the range of operation of said uni-control device, said latter means constituting a feedback coupling element common to the anode and grid circuits of said oscillator.

3. In a signal receiving arrangement, a signal frequency amplifier including a coupling system, a modulator responsive to the output of said arnplifier and a vacuum tube oscillator having a cathode, anode and grid and circuits connected therewith, a first variable capacity for tuning said oscillator over a range in frequency, a second variable capacity included in said coupling system for tuning said system to the signal frequency, uni-control means for simultaneously operating said first and second variable capacities, and means for maintaining a constant frequency difference between the frequency to which said coupling system is tuned and the frequency to which said oscillator is tuned when said uni-control means is adjusted over its range of operation comprising a fixed capacity connected to couple the grid and anode circuits.

4. In a signal receiving arrangement, a coupling system, a modulator couple-d to said coupling system, a vacuum tube oscillator having a cathode, anode and grid and circuits connected therewith, said oscillator being coupled to said modulator, said anode and grid circuits being coupled by inductance means and by a capacity, said inductance means and capacity serving to produce sustained oscillations, a variable capacity for tuning said oscillator over a range in frequency, a variable capacity for tuning said coupling system over a range in frequency, uni-control means for simultaneously operating said variable capacities so that there is a difference between the frequency to which said coupling system is tuned and the frequency to which said oscillator is tuned, said oscillator coupling capacity being proportioned with respect to said variable capacities so that said frequency difference is maintained substantially constant over the range of operation of said uni-control means.

5. A heterodyne receiving system including a tuned signal frequency circuit and rectifying and oscillating means coupled thereto, said oscillating means including a vacuum tube having a.

tuned input circuit and an output circuit, a unicontrol device for simultaneously adjusting the tuning of said signal frequency and oscillator circuits. capacitive reactance means for maintaining a fixed frequency difference between the frequency to which said signal frequency circuit is tuned and the frequency to which said oscillator is tuned over therange of operation of said uni-control device, said capacitive reactance vmeans constituting a coupling element common to the input and output circuits of'said vacuum 6. In a radio receiver, in combination, means for receiving radio signals, a frequency selecting system including a variable element for tuning said selective system, a local oscillator system and a modulator for modulating the radio signals with the oscillations from said oscillator, said oscillator system comprising a vacuum tube having a cathode, a grid and an anode and circuits connected thereto, and having a frequency-determining circuit, the grid and anode circuits being coupled to said frequency-determining circuit both capacitively and magnetically, a variable tuning element for said frequency-determining circuit, and an output circuit from said oscillator coupled to a circuit of said modulator, the variable element of said selective circuit being o-perated in conjunction with the variable element of said frequency-determining circuit by uni-control arrangement, and said oscillator coupling and said variable elements being so related that there is a iixed frequency difference between the frequency to which said selective circuit is tuned and the frequency to which said oscillator frequency-determining circuit is tuned.

7. In a signal responsive system, a signal selective circuit, a modulator and a vacuum tube oscillator having a cathode, anode and grid and circuits connected therewith, said oscillator having associated therewith a frequency-determining circuit for determining the frequency of oscillations, means for tuning said signal selective circuit and said frequency-determining circuit over a range in frequency, a uni-control device for simultaneously adjusting the tuning of said signal selective circuit and of said frequency-determining circuit and means for maintaining a fixed frequency difference between the frequency to which said signal selective circuit is tuned and the frequency to which said frequency-determining circuit is tuned over the range ofoperation of said uni-control device, said latter means constituting a capacitive coupling element common to the grid and anode circuits and the frequency-determining circuit of said oscillator.

8. In a signal receiving arrangement, a tunable coupling system, ainodulator anda vacuum tube oscillator having a cathode, anode and grid and circuits connected therewith, a first variable capacity for tuning said oscillator over a range in frequency, a secondvariable capacity included 'ir'i` said coupling system for tuning said system to i thesignal frequency, uni-control means for simultaneously operating said first and second variable capacities and a fixed capacity common to said grid and anode circuits and the tuning circuit of said oscillator for maintaining a constant frequency difference between ther frequency to which said coupling systemr is tuned. and the frequency to ywhich said oscillator is tuned when saidv uni-control means `is adjusted over its range of operation.

75 pling system, a modulator coupled to the output 9. In a signal receiving arrangement, a couof said coupling system, a vacuum tube oscillator having a cathode, anode and grid and circuits connected therewith, and having a frequencydetermining circuit, the output of said oscillator being coupled to a circuit of said modulator, said grids, anode and frequency-determining circuits `including common inductancemeans and a common capacity, a variable capacity for tuning said frequency-determining circuit over a range in frequency, a. variable capacity for tuning said coupling system over a range in frequency, unlcontrol means for simultaneously operating said variable capacities so that there is a difference between the frequency to which said coupling system is tuned'and the frequency to which said oscillator frequency-determining circuit is tuned, said oscillator coupling capacity being proportioned with respect to said variable capacities so that said frequency difference is maintained substantially constant over the range of operation of said uni-control means.

l10. In radio reception, the method of reception which comprises tuning in a receiving system to signal-representing oscillations throughout a range of frequencies thereof by varying a capacity, locally generating oscillations by coupling the output system of a thermionic tube to its input system magnetically and capacitatively through a capacity common to said output and said input systems, through an inductive coupling impressing upon said receiving system locally generated oscillations, by said magnetic and capacitative couplings rendering constant throughout the frequency range the voltage of the oscillations so impressed on said receiving system, controlling the frequency of the generated oscillations by varying another capacity in said ,input system, and utilizing said capacity common to said ouput and input systems to modify the action of said last named -variable capacity in series therewith in said input system to cause the -variable capacities of said receiving system and said input unison to effect throughout the frequency range a constant dilerence between the frequencies of the locally generated and signal-representing oscillations.

11. The combination with a system traversed by oscillations, said system comprising a circuit including inductance and a variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations', means for impressing upon said system. oscillations produced by said generator, said generator comprising a thermionic tube having input and output systems, means for magnetically coupling said input and output systems of said tube, a capacity for coupling said input and output systems of said tube, said magnetic and capacitative couplings between said output and input systems being sorelated that the voltage of the oscillations impressed upon said first named system by said means remains constant throughout the Vfrequency range of the locally generated oscillations, one of said systems of said tube comprising an inductance and said coupling capacity in series with a capacity Variable to determine the frequency of the generated oscillationssaid coupling and last named variable capacity being so related that said variable capacities of said first named system and of one'of said tube systems shall be adjustable in unison to effect throughout the frequency range a constant difference between the frequencies of the oscillations in said system to be adjustable in rst named system and the locally generated oscillations.

12. A heterodyne receiving system comprising a circuit including inductance and variable capacity for tuning it to signal-representing oscillations throughout a range of frequencies thereof, a generator of oscillations comprising a thermionic tube, means for magnetically coupling the output to the input system of said tube, a capacitative coupling between said output and said input systems comprising a capacity common to them, an inductive coupling for impressing locally generated oscillations upon said receivingr system, said magnetic and capacitative couplings between said output and said input systems rendering constant throughout their frequency range the voltage of the oscillations impressed on said receiving system, said capacity common to said output and input systems disposed in series with a variable frequency determining capacity in one of said systems of said tube to cause said variable capacitiesf said circuit and said one of said tube systems to be adjustable in unison to effect throughout the frequency range a constant differance between the frequencies of the locally generated and signal-representing oscillations.

13. A heterodyne receiving system comprising a circuit including inductance and variable capacity for tuning it to signal-representing oscillations throughout a range of frequencies thereof, a generator of oscillations comprising a thermionic tube, means for magnetically coupling the output to the input system of said tube, a capacitative coupling between said output and said input systems comprising a capacity common to them, an inductive coupling for impressing locally generated oscillations upon said receiving system, said magnetic and capacitative couplings between said output and input systems rendering constant throughout their frequency range the voltage of the oscillations impressed on said receiving system, means for mechanically coupling the adjustable elements of said variable capacities of said circuit and said input system, and means for insuring constant difference between the frequencies of the locally generated and signal-representing oscillations during adjustment in unison of said adjustable elements comprising said capacity common to said output and input systems disposed in series in said input system with said variable capacity thereof.

14. A heterodyne receiving system comprising a circuit including inductance and variable capacity for tuning it to signal-representing oscillations throughout a range of frequencies thereof, a generator of oscillations, an inductive coupling for impressing locally generated oscillations upon said receiving system, said generator comprising `a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and a plurality of additional capacities all in series with each other in circuit with said second named inductance, connections from the terminals of a portion of the capacity of said input system to input electrodes of said tube, a magnetic coupling between said output and input systems, and a capacitative coupling between said output and input systems comprising one of said plurality of additional capacities of said input system, said plurality of additional capacities in series with each other having a combined capacity so related to the maximum capacity of said variable capacity of said input system that the adjustable elements of said variable capacities of said circuit and said input system may be adjusted in unison throughout the frequency range while maintaining constant the difference between the frequencies of said locally generated and signal-representing oscillations, said magnetic and capacitative couplings between said output and input systems rendering constant throughout their frequency range the voltage of the oscillations impressed upon said receiving system.

15. The combination with a system traversed by oscillations, said system comprising a circuit including inductance and variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations, means for impressing upon said system oscillations produced by said generator, said generator comprising a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and another capacity in series therewith in circuit with said last named inductance, connections to the input electrodes of said tube from the terminals of one of said last named capacities, a third inductance coupled to said second named inductance, a connection from an anode to a cathode of said tube through said third inductance and said other of said capacities, whereby the output and input vsystems of said tubes are magnetically and capacitatively coupled, and means mechanically coupling the adjustable elements of the variable capacities of said circuit and said input system for adjusting them in unison.

16. The combination with a system traversed by oscillations, said system comprising a circuit including inductance and variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations, means for impressing upon said system oscillations produced by said generator, said generator comprising a thermionie tube having output and input systems, said input system including an inductance, a variable capacity and a plurality of additional capacities all in series with each other in circuit with said second named inductance, connections from the terminals of part of the capacity of said input system to the input electrodes of said tube, and means for magnetically and capacitatively coupling the output to the input systems of said tube comprising a connection from an anode to a cathode of said tube through a point between two of said plurality of capacities of said input system, and a third inductance in said connection coupled to said second named inductance.

17. The combination with a system traversed by oscillations, said system comprising a circuit including inductance and variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations, means for impressing upon said system oscillations produced by said generator, said generator comprising a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and a plurality of additional capacities all in series with each other in circuit with said second named inductance, connections from the terminals of a part of the capacity of said input system to the input electrodes of said tube, means for capacitasaid tube comprising a connection from an anode tively coupling the output to the input systems of to a cathode of said tube through a point between two of said plurality of capacities and through one of them, Va third inductance in said connection coupled to said second named inductance prising a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and another capacity in series with each other in circuit with said inductance, a connection from a terminal of said variable capacity and said inductance to a control grid of said tube, a connection from a cathode of said tube to a point between' said serially connected capacities of said input system, a third inductance coupled to said second named -inductance, and a connection from an anode of said tube to said cathode through said third inductance and a point between said serially connected capacities of said input system.

, including inductance and variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations, means for impressing upon said system oscillations produced by said generator, said generator comprising a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and another capacity in series with each other in-circuit with said second named inductance, a connection from a terminal of said variable capacity and said inductance to a control grid of said tube, a connection from a cathode of said tube to a point between said serially connected capacities of said input system, a third inductance in the output system of said tube coupled to said second named inductance, and a resistance connected between the other terminal of said second named inductance and said cathode.k

20. The combination with a system traversed by oscillations, said system comprising a circuit including inductance andv variable capacity for tuning to said oscillations throughout a range of frequencies thereof, a generator of oscillations, means for impressing upon said system oscillations produced by said generator, said generator comprising a thermionic tube having output and input systems, said input system including an inductance, a variable capacity and a plurality of capacities in series therewith and with each other in circuit with said second named inductance, connections from the terminalsv of a part of the capacity of said input system to the input electrodes of said tube. a connection from one terminal of said second named inductance to one of said input electrodes, a resistance connected between the other terminal of said second named inductance and a cathode of said tube, a third inductance coupled to said second named inductance, and a connection from an anode of said tube to a cathode thereof through'said third inductance to. a point between a pair of said capacfrequencies thereof, a generator of oscillations, means for impressing upon said system'oscilla-- tions produced by sald generator, said'generator comprising a thermionic tube having output and inputl systems, said input system including an in- Y,

ductance, a variable capacity and a plurality of capacities in series therewith and witheach other connections from theterminalslof a part of the capacity' of said input system to input' electrodesy .in circuit with said second' named inductance,

of said tube, a connection from one terminal'of said second named inductance to one of said input electrodes, va resistance connected between the other terminal of said second named inductance,

thereof through saidl third inductance to a point between a pair of said capacities of said input system, and means for mechanically coupling the adjustable elements of said variable capacities of said circuit and said input system.

22. In a heterodyne receiver, a system traversed by signal-oscillations comprising at least one circuit tunable by a variable condenser through a range of signal frequencies, andan oscillator system inductively coupled to said iirst system and comprising a tuning condenser adjustable through substantially the same range of capacity as said ilrst condenser, characterized by the fact that the input and output systems of the oscillator tube are inductively and capacitatively coupled, the coupling capacity having such magnitude that theA Voltage of the locally generated oscillations as impressed upon the signal receiving system is substantially 'constant throughout the range of frequency adjustment of the oscillator, and that throughout the range of adjustment of said similar condensers irr unison, the beat frequency between signal oscillations and locally produced oscillations is'constant.

23. In a heterodyne receiver, a system traversed by signal oscillations comprisingat least one circuit tunable by a variable condenser through a range of signal frequencies, a local oscillator system including a condenser adjustable through substantially the same range of capacity as said first condenser, means for cou.

pling said oscillator system to said `first system having the characteristic'that the voltage of the locally generated oscillations as impressed upon said rst system increases Withv increase of frequency, and a ilxed condenser for coupling the input and output circuits of the oscillator tube of such magnitude that the voltage of the locallyy generated oscillations decreases with increase of frequency .compensating for the rising characteristic of said coupling means, and that through the range of adjustment of said similar condensers 'in unison, the-beat frequency between signal oscillations and locally produced oscillations is constant.

24. In a heterodyne .receiving system comprising a system traversed by signal oscillations inductively coupled to a local oscillator, the oscillator and said system having tuning condensers adjustable in unison through the same range of capacity, the method of reception which comprises introducing into the coupling between' the input and output circuits of the oscillator a ca.- pacitative impedance of such magnitude that the voltage of the locally produced oscillations as impressed upon said system is independent of their frequency, and that throughout the range of adjustment of said similar condensers, the beat frequency between the signal oscillations and the locally produced oscillations is constant.

25. In a signal receiver, a signal selector having a variable tuning element, modulator means upon which are impressed signals from the output of said selector, vacuum tube oscillator means including ay cathode, a control grid, and an anode for generating local oscillations to be combined with incoming signals by said modulator means, said anode and grid being inductively and capacitively coupled to provide feedback paths therebetween, a variable tuning element for adjusting the oscillation frequency of said ,oscillator means over a frequency range, uni-control means for varying simultaneously both said tuning elements, and means including capacity in one of said feedback paths for maintaining substantially constant the difference between the frequency to which said oscillator means is tuned and the frequency to which said selector is tuned.

26. In a signal receiver, a tunable signal selector having a variable tuning condenser, modulator means upon which incoming signals are impressedfrom the output of said selector, vacuum tube oscillator means including a cathode, a control grid. and an anode, coupling means including coupling inductance and coupling capacity for providing feedback paths between said anode and grid to produce local oscillations, said coupling capacity being adapted to produce' greater feedback effect at lower frequencies and said coupling inductance being adapted to produce greater feedback effect at higher frequencies, means including said modulator means for combining said local oscillations with said incoming signals, a variable tuning condenser for adjusting the oscillation frequency of said oscillatorover a frequency range, uni-control means for varying simultaneously both of said tuning condensers, and means including said coupling capacity for maintaining substantially constant the difference between the frequency to which said selector is tuned and the frequency to which said oscillator means is tuned. Y

27. In a superheterodyne receiver, a signal selector having a variable tuning element, a local oscillator having a variable tuning element for adjusting the oscillation frequency thereof, a unicontrol device for varying simultaneously said tuning elements, modulator means for combining signals from said signal selector with local oscillations from said oscillator, said oscillator including vacuum tube means having a cathode, a

grid, and an anode and circuits therefor, said anode and grid being coupled both magnetically and capacitively to provide feedback paths therebetween, said capacitive coupling including a condenser in the oscillator cathode circuit common to the grid and anode circuits thereof, said condenser being so proportioned as to maintain a fixed frequency difference between the frequency to which said signal selector is tuned and the frequency to which said local oscillator is tuned.

28. In a signaling system, a signal selector having a variable tuning element, said signal selector being coupled to vacuum tube oscillator means including a cathode, a control grid, and an anode, said grid and anode being inductively and capacitively coupled to provide feedback paths therebetween, a variable tuning element `for adjusting the oscillation frequency of said oscillator means over a frequency range different from that of said signal selector, uni-control means for Varying simultaneouslyboth of said tuning elements, and means including capacity in one of said feedback paths for maintaining substantially constant the difference between the frequency to which said oscillator means is tuned and the frequency to which said selector is tuned.

29. In a signal receiver, a signal selector having a variable tuning element, modulator means upon which are impressed signals from the output of said selector, vacuum tube oscillator means including a cathode, a control grid circuit, and an anode circuit for generating local oscillations to be combined with incoming signals by said modulator means, said grid circuit including a loop circuit having as elements a coupling capac- I ity, a coupling inductance and a variable tuning capacity, said anode circuit including said coupling capacity and a coupling inductance magnetically coupled to said first-mentioned inductance, whereby said grid and anode circuits are capacitively and inductively coupled to provide feedback paths therebetween, said tuning capacity being variable to adjust the oscillation frequency of said oscillator means over a frequency range, uni-control means for varying simultaneously said tuning element and said tuning capacity, and means including said coupling capacity for maintaining substantially constant the difference between the frequency to which said oscillator means is tuned and the frequency to which said selector is tuned.

WILLIAM A. MACDONALD.

CERTIFICATE or CORRECTION..

mem No. 2,097,253. my 9, 193s.

WILLIAM A. MacDoNALD.

It s hereby certified that error appears in the printed specification of the above numbered patent requiring correction-as follows: Page 2, first column, line 47, after "frequency" insert a comma; second column, line 60, insert a comma after "trode" and strike out the comma after "grid"; page 3, first column, line 15, for "oscillation" read oscillations; page4, first column, line 23, for "supply"read apply; page 5, first column, line 48, for "received'bread receiver; and line 54, for "amplimy" read amplify; page 6, second column, line 3, for "circuit" read circuits; page 8, second column, line 45, forI "1200" read 120;' page 11, second column, line 6, claim 9, for "grids" read grid; .and line 39, claiml 10, for "ouput" read output; page 12. second column, line 7l, `claim 17, strike out the syllable and words "tively coupl ing the output tothe input systems of" and insert the same after line 69. of said claim; and that the said Letters Patent should be read with these corrections therein that the same may conform to the record of the case in the Patent Office.

Signed and sealed this 27th day of August, A. D. -1935.

Leslie FCrazer (Seal) Acting Commissioner of Patents. 

